The present invention relates to a coating method and apparatus, and more particularly a method of and an apparatus for coating surfaces such as side panels, an engine hood, a roof, etc., of an automotive body, for example, with a plurality of paint spray guns in a short period of time and with high accuracy.
The automobile industry in recent years employs highly automated line production processes for efficiently mass-producing automobile products. There are used considerably automated coating apparatus devices for applying desired paint coatings to automotive bodies.
Various methods have heretofore been employed for automatically painting or coating automotive bodies. One known coating method is disclosed in Japanese Patent Publication No. 54-37972, for example.
A coating apparatus for carrying out the coating method disclosed in the above publication is illustrated in FIG. 1 of the accompanying drawings. The coating apparatus, generally indicated by the reference numeral 2, includes a portal frame 4, a pair of side coating mechanisms 6a, 6b disposed on sides of the legs of the frame 4 and an upper coating mechanism 8 mounted on an upper wall of the frame 4. The side coating mechanisms 6a, 6b have paint spray guns 10a, 12a and 10b, 12b, respectively, which are fixed to arms 16a, 16b movable along arcuate guide grooves 14a, 14b defined in the side coating mechanisms 6a, 6b. Although not shown, the upper coating mechanism 8 also has paint spray guns identical to the paint spray guns 10a, 12a and 10b, 12b.
When an automotive body W is delivered at a certain speed by a conveyor 18 into the portal frame 4, the side coating mechanisms 6a, 6b and the upper coating mechanism 8 are operated to displace the arms 16a, 16b along the arcuate guide grooves 14a, 14b. At the same time, different paint sprays are ejected from the paint spray guns 10a, 12a and 10b, 12b on the arms 16a, 16b onto side panels of the automotive body W in overlapping patterns, and paint sprays are also ejected from the paint spray guns of the upper coating mechanism 8 onto the roof of the automotive body W, thus coating the automotive body W.
In the prior coating apparatus 2, the arms 16a, 16b with the paint spray guns 10a, 12a and 10b, 12b are displaceable along the guide grooves 14a, 14b which are shaped to match the configuration of the side panels of the automotive body W. If an automotive body W of a different type or shape is to be coated with the coating apparatus 2, however, the paint spray guns 10a, 12a and 10b, 12b cannot be displaced accurately along the side panels of the automotive body W, resulting in a coating failure or trouble. Therefore, when an automotive body W of a different design is to be coated accurately, another plurality of coating mechanisms 6a, 6b having paint spray guns 10a, 12a and 10b, 12b displaceable along the shape of the automotive body W must be employed. This is disadvantageous in that the cost of the coating apparatus required to coat automotive bodies having different configurations is high.
The side panels of the automotive body W are coated by moving the paint spray guns 10a, 12a and 10b, 12b from lower to upper ends of the side panels. It is thus necessary to cause the conveyor 18 to deliver the automotive body W at a considerably low speed. This coating process is, however, low in efficiency.
Paint sprays of one type may be applied to the automotive body W from the paint spray guns 10a, 12a. If the automotive body W is coated by an electrostatic painting process, however, the particles of the paint sprays ejected from the paint spray guns 10a, 12a which are located closely to each other are repelled from each other, and the automotive body W cannot be coated highly accurately.
Another apparatus for automatically coating an automotive body is disclosed in Japanese Laid-Open Utility Model Publication No. 57-25758, for example.
The disclosed coating apparatus has a plurality of paint spray guns mounted on a frame extending transversely across the automotive body and displaceable transversely and vertically with respect to the automotive body. While the automotive body is being fed at a given speed, the frame is vertically moved along the upper surface configuration of the automotive body and also reciprocally moved transversely across the automotive body. At the same time, paint sprays are ejected from the paint spray guns to coat the upper surfaces of the automotive body.
The upper surfaces of the automotive body to be coated include an engine hood, pillars, a roof, and a trunk lid, the roof usually having a width or transverse dimension smaller than those of the engine roof and the trunk lid. The paint spray guns are positioned to cover the surface having the maximum width, e.g., the engine hood. The engine hood, the pillars, the roof, and the trunk lid are coated by the paint spray guns which they are being displaced in unison. Because the surfaces to be coated have different widths, however, when the roof is coated by the paint spray guns which have been positioned to cover the engine hood, the paint spray guns are displaced toward positions where no coating is required as the roof is narrower than the engine hood. More paint than is necessary to coat the roof is thus applied to the roof by the paint spray guns. Such a coating process is quite uneconomical due to the wasteful consumption of paint.
When coating narrow and slanted surfaces such as the pillars, it is difficult to move the paint spray guns precisely along these surfaces. While these surfaces are being coated, a coating failure tends to occur, and more paint than necessary is consumed.
In the latter coating apparatus, a mechanism for displacing the frame with the paint spray guns thereon back and forth in the transverse direction of the automotive body is usually in the form of a crank mechanism since it is simple in structure and small in size. The crank mechanism includes a rotor coupled to a rotative drive source such as a motor and a rod having one end joined eccentrically to the rotor and the other end connected to the frame. By rotating the rotor with the rotative drive source, the frame is moved reciprocally to displace the paint spray guns back and forth in the transverse direction of the automotive body.
When the paint spray guns are displaced back and forth by the crank mechanism, the paint spray guns are moved at a speed V indicated as follows: EQU V=.omega..gamma.sin.omega..gamma. (1)
where .omega. is the angular velocity of the end of the rod which is joined to the rotor and .gamma. is the radius of curvature of a circular path along which said end of the rod moves. As indicated by the above equation, each paint spray gun moves along a sine curve. Consequently, as shown in FIG. 2, each paint spray gun, denoted at 20, is moved at a higher speed in the central portion of its stroke than at the opposite ends of the stroke. If a paint spray 22 is ejected from the paint spray gun 20 uniformly with respect to time, opposite ends 26a, 26b of a paint coating 26 applied to a surface 24 are raised, i.e., higher than the rest of the paint coating 26. Accordingly, the paint coating 26 cannot be applied in a uniform thickness, and hence is either low in quality or defective.
Japanese Patent Publication No. 58-22262 or Japanese Laid-Open Utility Model Publication No. 62-87762 shows still another apparatus for automatically coating an automotive body. According to this prior coating apparatus, a portal frame extends transversely across the automotive body and is displaceable in the transverse and vertical direction with respect to the automotive body, and a plurality of paint spray guns are mounted on the frame. While the automotive body is being fed at a certain speed, the paint spray guns apply paint coatings to surfaces of the automotive body.
In general, the coating apparatus is placed in a coating booth in which an air flow is developed vertically downwardly. The downward air flow forcibly lowers excessive paint particles to prevent them from being applied to the automotive surfaces.
The frame is of a portal shape and its horizontal member has a relatively large prismatic shape. Therefore, the air flow hits the horizontal surface of the frame and becomes a turbulent air flow. As a consequence, paint particles are caused by the turbulent air blow to be suspended in the coating booth and eventually applied to the automotive body, or even to enter the mechanism of the coating apparatus, thereby obstructing smooth operation of the coating apparatus.